In recent years, a radiation image recording and reproducing method employing stimulable phosphor has been utilized in place of a radiographic method employed in combination with a conventional radiographic film and an intensifying screen.
In the method, a radiation image conversion panel (also called a phosphor-accumulating sheet) comprising a stimulable phosphor is employed, and the method comprises the steps of causing the stimulable phosphor of the panel to absorb radiation having passed through an object or having been radiated from an object, sequentially exciting the stimulable phosphor with an electromagnetic wave such as visible light or infrared rays (hereinafter referred to also as “stimulating light”) to release the radiation energy stored in the phosphor as light emission (stimulated emission), photo-electrically detecting the emitted light to obtain electrical signals, and reproducing the radiation image of the object as a visible image from the electrical signals. The panel, having been read out, is then subjected to image-erasing and prepared for the next photographing cycle. Thus, the radiation image conversion panel can be used repeatedly.
In the radiation image recording and reproducing method described above, a radiation image is advantageously obtained with a sufficient amount of information by applying radiation to an object at a considerably smaller dose, as compared to the conventional radiographic method. Further, in conventional radiography, the radiographic film is consumed for every photographing; on the other hand, in this radiation image converting method, in which the radiation image conversion panel is employed repeatedly, is also advantageous in terms of conservation of resources and overall economic efficiency.
The radiation image conversion panel employed in the radiation image recording and reproducing method basically comprises a support and provided thereon a phosphor layer (stimulable phosphor layer), provided that, in cases where the phosphor layer is self-supporting, the support is not necessarily desired. The stimulable phosphor layer comprises a stimulable phosphor dispersed in a binder. There is also known a stimulable phosphor layer, which is formed by vacuum evaporation or a sintering process, free from a binder, and which comprises an aggregated stimulable phosphor. There is further known a radiation image conversion panel in which a polymeric material is contained in the openings among the aggregated stimulable phosphor. In addition, a protective film composed of a polymeric film or an evaporated inorganic film is provided on the surface on the stimulable phosphor support side and on the surface on the opposite side.
The stimulable phosphor, after being exposed to radiation, produces stimulated emission upon exposure to the stimulating light. In practical use, phosphors are employed, which exhibit an emission within a wavelength region of 300-500 nm stimulated by stimulating light of wavelengths of 400-900 nm. Examples of such stimulable phosphors include rare earth activated alkaline earth metal fluorohalide phosphors described in Japanese Patent O.P.I. Publication Nos. 59-56479 and 59-56480; bivalent europium activated alkaline earth metal fluorohalide phosphors described in Japanese Patent O.P.I. Publication Nos. 61-235486 and 61-235487; rare earth element activated oxyhalide phosphors described in Japanese Patent O.P.I. Publication No. 55-12144; cerium activated trivalent metal oxyhalide phosphors described in Japanese Patent O.P.I. Publication No. 58-69281; bismuth activated alkaline metal halide phosphors described in Japanese Patent O.P.I. Publication No. 60-70484; bivalent europium activated alkaline earth metal halophosphate phosphors described in Japanese Patent O.P.I. Publication Nos. 60-141783 and 60-157100; bivalent europium activated alkaline earth metal haloborate phosphors described in Japanese Patent O.P.I. Publication No. 60-157099; bivalent europium activated alkaline earth metal hydrogenated halide phosphors described in Japanese Patent O.P.I. Publication No. 60-217354; cerium activated rare earth complex halide phosphors described in Japanese Patent O.P.I. Publication Nos. 61-21173 and 61-21182; cerium activated rare earth halophosphate phosphors described in Japanese Patent O.P.I. Publication No. 61-40390; bivalent europium activated cesium rubidium halide phosphors described in Japanese Patent O.P.I. Publication No. 60-78151; bivalent europium activated cerium halide rubidium phosphors described in Japanese Patent O.P.I. Publication No. 60-78151; and bivalent europium activated composite halide phosphors described in Japanese Patent O.P.I. Publication No. 60-78153.
It is known that iodide-containing bivalent europium activated alkaline earth metal fluorohalide phosphors, iodide containing rare earth metal activated oxyhalide phosphors and iodide containing bismuth activated alkaline earth metal halide phosphors specifically exhibit high sensitivity stimulated luminescence.
Along with the spread of radiation image conversion panels employing stimulable phosphors is further desired an enhancement of radiation image quality, such as enhancement in sharpness and graininess. Especially in recent years, stimulable phosphor exhibiting high luminance and reduction of sharpness drop caused by X-ray damage has been demanded.
The preparation method of stimulable phosphor is called a solid phase process or calcination method, in which pulverization after calcination is indispensable, however, there were problems such that it was difficult to control the particle form affecting sensitivity and image performance. Of means for enhancing image quality of radiation images is valid preparation of fine particles of a stimulable phosphor and enhancing particle size uniformity of the fine stimulable phosphor particles, i.e., narrowing the particle size distribution.
Preparation of stimulable phosphors in the liquid phase described in Japanese Patent O.P.I. Publication Nos. 7-233369 and 9-291278 is a method of obtaining a stimulable phosphor precursor in the form of fine particles by adjusting the concentration of a phosphor raw material solution, which is valid as a method of preparing stimulable phosphor powder having a narrow particle size distribution. Of rare earth activated alkaline earth metal fluorohalide stimulable phosphors, a phosphor having higher iodide content is preferred in terms of reduction of radiation exposure. This is due to the fact that iodine exhibits a higher X-ray absorption than bromine. That is, alkaline earth metal fluoroiodide stimulable phosphors prepared in the liquid phase are advantageous in luminance and graininess.
To enhance the yield of a rare earth activated alkaline earth metal fluorohalide stimulable phosphor, specifically, an alkaline earth metal fluoroiodide stimulable phosphor, there is disclosed a method for obtaining cubic or rectangular rare earth element-containing barium fluoroiodide crystals having a basic composition of BaFI:xLn (in which Ln: is at least a rare earth element selected from Ce, Pr, Sm, Eu, Gd, Tb, Tm and Yb, 0<x≦0.1) which is obtained by adding a fluorine source to the mother liquor and concentrating the solution (Japanese Patent O.P.I. Publication No. 11-29324).
A method of preparing an oxygen-introduced rare earth activated alkaline earth metal fluorohalide stimulable phosphor having a small particle diameter together with a narrow particle diameter distribution is described in Patent Documents 1 and 2.
However, along with active utilization of the radiation image conversion method employing stimulable phosphor, further demanded have been improved image quality of the resulting radiation images together with, for example, improved luminance and graininess and reduction of luminance degradation caused by X-ray damage through repetitive use.
Patent Document 1: Japanese Patent O.P.I. 2002-38143
Patent Document 2: Japanese Patent O.P.I. 2003-268369